Re: [Qemu-block] NBD structured reads vs. block size

[Richard W.M. Jones]

On Wed, Aug 01, 2018 at 04:49:21PM +0300, Nir Soffer wrote:
> All this is an issue only when the underlying file is a file using raw
> format, right?

NBD should only really be used to send raw format data over the wire.
While it is possible to serve a qcow2 file over NBD as qcow2, it's not
really how NBD is intended to be used.

> Since NBD is meant for block devices, and block devices cannot have
> partial blocks, would it be simpler to round down unaligned file size,  and
> treat possible unaligned range at the end of the file as area that can not
> be accessed via NBD?

For nbdkit I would like to preserve the ability to serve arbitrary-
sized files.  nbdkit allows people to write plugins, and we cannot be
sure that every plugin everyone writes will always use sensible block
sizes.

In a forthcoming nbdkit release you can add the "truncate" filter on
top of any plugin to round up the size to a multiple of any power of 2
(or round down if you really want).

https://www.redhat.com/archives/libguestfs/2018-August/msg00006.html

Rich.

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